Studies are proposed to examine the development of cytotoxic leucocytes during the course of experimental Chages' disease against 1) cells infected with Trypanosoma cruzi and 2) normal syngeneic, allogeneic and xenogeneic target cells. The phenomena of development of parasite-antigen specific and tumor cell-antigen specific cytotoxic T lymphocytes as well as natural killer cells and cells presenting spontaneous lytic activity during the course of infection will be studied. It is thought that elucidation of the activities of cytotoxic leucocytes may be important in understanding immune resistance as well as immunopathology in Chagas' disease.